Edit the GameSettings.script file in your install directory using Notepad. Replace the following line:
bool ConsoleEnabled = false;
with
bool ConsoleEnabled = true;

Cheat Codes
Press [F3] to activate the console once in the game, then type any of the following codes:
Code - Result:
immortal = true - Immortality
god_mode = true - God Mode
unlimited_ammo = true - Unlimited Ammo
fullammo - Full Ammo
enabled_missions = true - Unlock All Missions